This is because a new extent was allocated and it was 128MB in size. The
real problem is revealed by investigating with dbms_space. Even after
deletion, DBMS_SPACE did not show any partially filled or empty blocks,
only full blocks and unformatted blocks. That means that delete doesn't
free blocks. Here is the script which invokes DBMS_SPACE that I used:

The block numbers before and after deletion are exactly the same. Either
DBMS_SPACE is buggy or space management within LOB segment is buggy.
Judging by the huge disk space consumption, the latter is the case.